1. Field of the Invention
The present invention relates to the field of teletypewriter type switching systems for remote signaling and communication purposes. Also, the present invention relates to an improved teletypewriter keyer circuit for transmitting serial data in the form of a current in a load to a terminal for remote signaling.
2. Prior Art
In certain applications, it is desired to transmit switching signals over a line or lines for remote signaling and data communication purposes. In such a system, the receiver commonly has predetermined operating characteristics in terms of voltage and current, these parameters, of course, being related by the impedance thereof. The line or lines over which the signal is to be transmitted from the source to the receiver, however, are of widely varying characteristics, dependent upon the nature of the system, the distances over which signaling is to be achieved, etc. By way of example, while some systems use two lines, many systems simply use a single line with an earth return, so that the impedance of the line or lines may vary tremendously with the distances, condition of the lines, etc., ranging from a relatively low impedance up to an open circuit. Obviously, while communication over an open circuit is not achievable, reliable communication over a line or lines having an impedance on the order of or even substantially larger than that of the receiver is desired. Therefore, establishing line voltage at the sending end will not assure the receipt of adequate power at the receiving end for operation of the receiver.
Historically, teletypewriter systems have used a relatively high voltage battery, typically having a voltage of a hundred volts or more, with a large resistor in series therewith. When the keyer is closed, the battery and resistor are coupled across the line, the resistor limiting the current to safe limits even for a particularly low impedance line. This type of passive current regulation has the disadvantage however, of dissipating large amounts of battery power in comparison to the power put onto the line, and has the further disadvantage of providing poor regulation as line impedances become large.
In more recent times, active series regulators have been used in place of the current limiting resistors so as to provide well regulated current sources for driving the lines substantially independent of the line impedance, at least for line voltages approaching the battery voltage. Such regulators have the advantage of being electrically quiet, i.e. low ripple in the output current, and operate well over a wide range of line impedances, through still dissipate a large amount of power in comparison to the transmitted power. Because of the high power dissipation, such systems are relatively expensive and may have reduced reliability because of the power dissipation therein.
In general, a "keyer" circuit is used to generate a predetermined current level in a resistive or inductive load that is located in standard teletypewriter terminals (TTY's) common in most Telex systems. Most TTY's require plus and minus 60 milliamps of load current to read a data bit. As previously mentioned to deliver the required load current, early designs of keyer circuits used high-voltage power supplies, each connected in series with a variable potentiometer and a mechanical relay switch to connect and disconnect, on command, the power supplies to the load. The setting or value of the variable potentiometer was used to control the amount of current that would flow in the TTY load. This early design suffered from several disadvantages, such as wasted power resulting from the voltage drop across the series potentiometer when the relay was connecting the load to the potentiometer, an electrically noisy environment created by the switching of the mechanical relay, and poor control of the amount of current in the load. Later designs of keyer circuits substituted solid state switching for the mechanical relay switch, to eliminate the slow response time and the inherently noisy electrical spiking that results when mechanical relays are used. However the disadvantages of poor current regulation in the load current, and power losses in the series potentiometer and solid state switch resulting from the high voltage power supplies' continuous connection to the load, were still present.
To solve the problem of current regulation, a series pass regulator was substituted for the series potentiometer such that the high voltage power supply was regulated to supply a constant current to the load. Unfortunately, these keyer designs still required a high power consumption from the power supply, because of the wasted power loss across the series pass regulator and the series switch. As in any large electronic system which dissipates power in the circuit elements, it was necessary to design efficient and costly cooling systems to remove the heat caused by this wasted power, as well as provide large and expensive power supplies to deliver this additional power.
Therefore, it would be advantageous to have a keyer circuit that would provide the requisite load currents but, at the same time, substantially reduce the amount of wasted power in the series elements connected between the power source and the load when "keying" data to the receiver.